This invention relates to a knock sensing apparatus for an internal combustion engine. More particularly, it relates to a knock sensing apparatus equipped with a fail-safe mechanism that ensures that the engine operates in a knock-free region even when a knock sensor of the knock sensing apparatus malfunctions.
Engine knocking is a form of engine vibration that occurs due to improper combustion in the cylinders of an engine. Prolonged knocking can damage an engine, so many engines are equipped with a knock sensing apparatus that senses engine vibrations and distinguishes vibrations due to knocking from other vibrations. When knocking is sensed, an operating parameter of the engine is adjusted so as to suppress the knocking. The most common method of suppressing knocking is to retard the ignition timing of the engine to the point that knocking no longer occurs.
A typical knock sensing apparatus includes a knock sensor in the form of an acceleration sensor that is mounted on the engine and generates an electrical output signal in response to engine vibrations. The output signal of the knock sensor is processed to separate components of the signal due to knocking from components due to miscellaneous mechanical vibrations and electrical noise. The ignition timing of the engine is then retarded until signal components due to knocking can no longer be detected.
The signal processing that must be performed on the output signal of the knock sensor is complicated, so a conventional knock sensing apparatus employs a considerable number of discrete electronic hardware components for carrying out the signal processing. The large number of components makes a conventional knock sensing apparatus bulky, and because of the labor required to assemble the components, the manufacturing costs are high. Furthermore, the characteristics of the individual hardware components of a conventional knock sensing apparatus can not be easily or widely adjusted, so the degree of freedom of control of the knock sensing apparatus is limited.